The step-by-step guide here shows how to apply makeup to look like a very grisly car-crash victim with injuries like a missing eye and exposed teeth. The costume definitely isn't for the faint of heart, but the actual process to put it on doesn't look that hard. The technique even includes a few common household items, but you might need a friend's help on some parts.
Going this route certainly shows more creativity than just buying a something from a store. Give the clip a watch to see how it's done.